1. Field of the Invention
The present invention relates generally to dissolution rate modifiers for photoresist compositions, and more specifically to dissolution rate modifiers comprising moderately low molecular weight molecules that have an acid labile pendant group.
2. Description of Related Art
For I-line (365 nm) and G-line (436 nm) irradiation, multicomponent positive-tone photoresists based on diazonaphthoquinone and novolac (DNQ-novolac) are commonly used in the electronics industry. In this case, DNQ acts as a dissolution rate modifier (DRM) for the novolac polymer binder or base resin. Thus, the unexposed areas of a positive acting (or positive tone) spin-coated film comprising DNQ-novolac is prevented from dissolving in developer, typically an aqueous solution of tetramethylammonium hydroxide (TMAH). However, upon irradiation, the DNQ undergoes a photochemical reaction forming, in the presence of water, a carboxylic acid derivative. This acid actually accelerates the dissolution of the positive acting novolac polymer binder resin in aqueous TMAH. This type of dissolution rate behavior can be expressed graphically as a function of added DNQ for both the exposed and unexposed areas of the film, referred to as a Meyerhofer plot in the art.
In the 1980's, multicomponent, chemically amplified positive tone photoresists based on poly(p-hydroxystyrene) (PHS) binder resin derivatives were developed in response to a general industry shift from I to G-line wavelengths of 248 nm in order to produce finer features and in response to productivity demands that called for more sensitive photoresists. The PHS binder resins are protected with acid labile groups that impart solubility to PHS in organic solvents such as propylene glycol methyl ether acetate (or PGMEA). Typically, this type of photoresist is formulated with a photoacid generator (or PAG), a compound which absorbs a photon, decomposes and eventually produces a proton. The proton then catalytically deprotects the acid labile group of the binder resin upon heating to form a deprotected binder resin and another equivalent of acid in the exposed regions of the wafer. The acid then deprotects a subsequent acid labile group. This process continues until many acid labile groups are deprotected. In this manner, the initial photochemical event is amplified chemically.
A wafer containing the photoreist is then developed with aqueous TMAH, which dissolves away the exposed regions providing a 3D relief pattern on the silicon wafer. One such exemplary PAG is triphenylsulfonium nonaflate. The sensitivity of such chemically amplified 248 nm positive acting photoresists was enhanced by incorporating a dissolution rate modifier (DRM) in the formulation. In a positive acting formulation, these DRM's act to suppress the dissolution rate of unexposed regions and ideally would enhance dissolution rates in exposed regions.
As the demand for smaller feature sizes grew in the late 1990's, the industry responded by developing chemically amplified photoresists for 193 nm exposure. Unfortunately, at this wavelength, PHS derivatives are too opaque. Four polymer binder resin platforms were identified that exhibited sufficient transparency at 193 nm to be considered as candidates. These were acrylate polymers, cyclic olefin/maleic anhydride copolymers, cyclic olefin/maleic anhydride/acrylate terpolymers, and vinyl addition cyclic olefin polymers. All of these polymers were developed as chemically amplified photoresists by appending acid labile groups to the backbone of the polymer. As in the case of the aforementioned 248 nm photoresists, DRM's used in 193 nm positive tone resist formulations suppress dissolution rates in the unexposed regions which limits dark film loss. Some examples of DRM's developed for such 193 nm photoresists include bile acid esters derived from cholic acid, deoxycholic acid, ursocholic acid and lithocholic acid. The aliphatic hydrocarbon nature of these compounds imparts high transparency at 193 nm and their multicyclic structure and low carbon-to-hydrogen ratios suggest that they should impart good dry etch resistance.
For the next generation of photolithography, exposure at 157 nm is most likely to be used. At this wavelength, the binder resins used at 193 nm are too opaque and, hence, unuseable. Likewise, DRM's such as cholic acid esters or multi-alicyclic compounds as described above will be too opaque. Some fluorinated compounds may be sufficiently transparent at 157 run to act as DRM's, for example the structures illustrated in FIG. 1.
An ideal DRM would meet the following criteria:                Suppress dissolution of the resist in the unexposed state        Increase dissolution of the resist in the exposed state        Lower or, at least, not increase optical density        Increase or, at least, not decrease etch resistance        Decrease or, at least, not increase surface roughness after etch        Be compatible with binder resin, i.e., not phase separate        
In addition, an ideal DRM would not suffer from volatilization from the photoresist film during post-apply bake or during post-exposure bake. However, introduction of fluorine in order to keep the optical density low, such as seen in the compounds above, in many cases increases the volatility of the compound.
It is generally known in the literature that chemically amplified resists, irrespective of the wavelength used, need to have the capability of being self-annealing. That is upon heating the photoresist film to a certain temperature, typically above its glass transition temperature, the film densities and decreases the amount of free volume present. This is important since films containing less free volume are less susceptible to air-borne base contaminants such as amines which can migrate through films and quench photochemically generated acids, thus shutting down the deprotection chemistry necessary for imaging. However, chemically amplified photoresist films cannot be heated near the decomposition temperature of the acid labile group. Thus, it is important that the glass transition temperature of the binder resin in the photoresist film be lower than the thermal decomposition temperature of the acid labile group (typically between 200° C. and 220° C.) on the binder resin. Glass transition temperatures of vinyl addition cyclic olefin polymers can be well over 300° C. Thus additives, such as DRM's, that lower the glass transition temperature of the photoresist binder resin would be desirable.
U.S. Pat. No. 6,124,074 to Varanasi et al. discloses acid-catalyzed positive photoresist compositions which are imageable with 193 nm radiation and are developable to form photoresist structures by the use of a combination of cyclic olefin polymers, photosensitive acid generators and hydrophobic non-steroidal multi-alicyclic components containing plural acid labile linking groups. The cyclic olefin polymers preferably contain i) cyclic olefin units having polar functional moieties, and ii) cyclic olefin units having acid labile moieties that inhibit solubility in aqueous alkaline solutions.
U.S. Pat. No. 6,136,499 to Goodall et al. discloses a radiation sensitive photoresist composition that includes a photoacid initiator and a polycyclic polymer comprising repeating units that contain pendant acid labile groups. Upon exposure to an imaging radiation source, the photoacid initiator generates an acid which cleaves the pendant acid labile groups, effecting a polarity change in the polymer. The polymer is rendered soluble in an aqueous base in the areas exposed to the imaging source.
However, none of the above-described photoresist compositions meet the criteria of an ideal DRM as set forth above when used with light at 157 nm. Thus, a need in the art exists for a photoresist composition that provides ideal DRM characteristics at 157 nm.